1. Field of the Invention
The present invention relates to a serial thermal printing method suitable for recording a half tone image.
2. Description of the Related Art
There are generally two thermal recording methods, including direct thermal printing (thermal recording) for directly recording an image on a recording paper and thermal transfer printing (thermal transfer recording) for transferring ink from an ink film to a recording paper. The thermal transfer recording has a thermal wax transfer type (melting type) for transferring melted ink to a recording paper and a thermal dye transfer type (sublimation type) which changes an ink amount transferred to a recording paper in accordance with a heat energy. For the thermal transfer recording, for example, a serial thermal printer is widely used in which a thermal head is moved in a subsidiary scan direction to record through one line and then the recording sheet is fed by one line in a main scan direction, in order to reduce the size and weight of the printer.
A thermal printing method using an area gradation method of recording a half tone by changing the area of ink dots in one pixel is known as described in, for example, Japanese Patent Laid-open Publication 5-155058. With this thermal printing method, one pixel is constituted by a plurality of main micro lines arranged in the main scan direction, and heating elements are selectively driven each time the thermal head is moved by one main micro line. One heating element selectively records an ink micro dot in one pixel at each main micro line. One ink dot is constituted by a plurality of micro dots. By changing the number of driving times of a heating element in accordance with image data representative of a tonal level, it is possible to change the area of micro dots in one pixel.
With the above-described thermal printing method, in recording of the first main micro line of the (n+1)th pixel after recording of the nth pixel, part of each heating element is positioned at a plurality of main micro lines which are the closing of the nth pixel. If the nth pixel is low in density and its closing main micro lines have no recording of micro dots, the density of the nth pixel is changed to middle by the recording of the (n+1)th pixel, because micro dots are recorded on these main micro lines. There is therefore a problem of a poor representation of gradation at a low density. If an applied voltage or conduction time of each heating element is changed to solve this problem, the gradation representation becomes poor at a high density.